This project proposes to elucidate the nature of the cellular clock, and to determine whether the timekeeping mechanisms which order synthetic events in dividing cells might also be involved in controlling gene expression and differentiation. Experiments will involve the isolation of specific messengers and their hybridization to temporally distinct fractions of DNA, with expectation that the time of replication of particular genes can be assessed in this manner. We are attempting to isolate tubulin mRNA and to determine whether the times of replication of genes coding for proteins which are expressed in fibroblasts differ from those proteins which are not expressed. A comparison of the time of replication of functional and non-functional genes in normal cells as opposed to transformed and neoplastic cells may indicate the role that altered order of replication plays in oncogenesis. As an adjunct to this study we have begun selectively mutagenizing limited numbers of temporally and biochemically identifiable genes by treatment with BUdR in different portions of S phase, and scoring the frequency of revertants in the population. BIBLIOGRAPHIC REFERENCES: Klevecz, R.R., Keniston, B.A. and Deaven, L.L. The Temporal Structure of S Phase. Cell 5: 195-203 (1975). Klevecz, R.R. and Forrest, G.L. Metabolic Regulation in the Cell Cycle. In: Nutrition and Metabolism of Tissue-Culture Cells, vol. III, eds. V.J. Cristofalo and G.H Rothblat, Academic Press, N.Y. (1976) in press.